“Flow control” is the ability to control a fluid in a way that makes it behave differently than it would naturally. This is a field that has been steadily rising in aerodynamics and fluid dynamics. Flow control may be separated into two different techniques: passive techniques (i.e., always acting on the fluid) and active techniques (i.e., which may be turned on and off or proportionally). Passive techniques are the older of the two techniques. Examples of passive techniques include vortex generators to enhance mixing, roughness strips to induce turbulence and air redirection to transport momentum. Active techniques may be more effective than passive techniques; however, they require energy input and are generally less developed as a technology. Examples of active techniques include synthetic jets and steady blowing (to inject momentum to the flow), as well as dynamic (i.e., active) roughness (to control the flow's turbulence levels).
The benefits of active roughness may be significant. Often, a flow control technique is only desired during certain periods of time. For example, flow control may be desired during takeoff and landing of an aircraft, and may not be desired while the aircraft is cruising. In addition, dynamic control of the fluid may allow for inducing various amounts of control of multiple actuators in the same flow field. For example, there may be two dynamic roughness elements, one on each wing of an aircraft. If one element is turned on while the other element is turned off, a non-symmetric lift is generated and may cause a roll action to the aircraft. This creates the ability to control the aircraft without the use of ailerons.
Examples of current dynamic roughness technologies include compressed air driven dimples, mechanically driven pistons, and electro active polymers (EAPs). Compressed air dimples, for example, are roughness elements that deflect utilizing controlled compressed air. Although current dynamic roughness technologies can actively change the roughness, and can achieve high levels of roughness displacement, deficiencies do exist.
With compressed air, the frequency response is extremely low and cannot be used to excite the flow's natural frequencies. To excite a flow field, different characteristic frequencies may be used. These frequencies can be on the order of about 100 Hz (for example, for a flow of air over a surface). Anything below this frequency will decay and not affect the flow field, which is undesirable. Anything well above this frequency may create a steady effect in time to the flow field. At high enough frequencies (on the order of about 1 kHz), the steady effect may be used (in conjunction with pulse or amplitude modulation) to excite characteristic frequencies. Another drawback to current designs is that fluidic plumbing to the actuator is required. This is a significant limitation of the technology and may not be desirable for use in an aircraft.
Mechanically driven roughness elements are typically applied in two ways, either the piston displaces fluid (which displaces dimples) or the piston itself acts as a roughness element. This form of roughness can achieve high deflections in both cases. However, the mechanical aspects of the piston tends to be very complex. Mechanically driven roughness elements may include an elaborate design and may be difficult to apply to a large scale surface. The designs also tend to be heavy and may penalize an aircraft (by added fuel expenditure). Also, a mechanical system may be limited to maximum frequencies that can be obtained (due to its structural design), and may have difficulty reaching the frequencies required to excite the flow field.
Another active roughness element developed is the EAP applied to a dimple configuration. Generally, EAPs are two surfaces that, when excited with a high voltage, are attracted to each another. With a dimple configuration between the two attractive surfaces, the result is a deflected dimple flow field. Although EAP technology does not have complex plumbing and may reach the frequencies of actuation required for flow excitement, the EAP has limited deflection capabilities. For example, EAPs typically reach maximum displacements on the order of about 0.1 mm. Also, EAPs are limited to deflect downwards into the surface. EAP technology generates an attractive force, so that dimples may start flat, deflect downwards into the surface, and then return to the flat position through one actuation cycle. In terms of energy consumption, EAPs require very high voltages (on the order of about 1,000 V). Lastly, a top surface of an EAP is electrified, which may require further development to make EAPs resilient to weather conditions, such as humidity and rain (which may cause outright failure of the actuator). It may be appreciated that human safety is also a factor with an electrified surface.